Genomic instability is an inherent characteristic of cancer and is the underlying reason for tumor progression as well as resistance to therapy. However, the precise molecular mechanisms causing genomic instability remain unclear. Here we provide evidence that Tcf-1 the T-cell specific DNA binding protein of the Wnt signaling pathway mediates maintenance of genomic integrity and that this function is antagonized by -catenin leading to chromosomally unstable T-cell lymphomas that have recurrent oncogenic translocations. In this regard my lab has made significant progress in identifying molecular processes that promote genomic instability. Both -catenin and Tcf-1 have essential functions at multiple stages of T-cell development and we have shown that deregulation of -catenin causes lymphomas. Lymphomagenesis is strictly RAG and c-Myc- dependent and requires TCR signaling. The lymphomas have recurrent RAG induced TCR/Myc translocations. These properties of the mouse model are relevant to human leukemia in several ways. Identical translocations have been seen in human T-cell leukemia. Mutations in -catenin and activation of Wnt signaling have been detected in genomically unstable T-cell leukemia/lymphoma, and it is known that lymphomas generated by conditional ablation of Pten or expression of active Akt have TCR/Myc translocations that depend on -catenin signaling. Our preliminary ChIP-seq analyses show that Tcf-1 co- occupies common DNA sites with the RAG2 recombinase. This is a particularly intriguing observation since we also found that DNA repair components compete with -catenin for binding to Tcf-1 and this results in altered repair of RAG induced DNA DSBs. Based on these findings we propose a model by which deregulated - catenin activity changes the chromatin landscape near Tcf-1 binding sites and alters the relative local availabilities of repair and recombination proteins. Simultaneously, synergy with TCR signaling enhances topological proximity of select loci that have sustained breaks, leading to chromosomal translocation. Our studies are designed to elucidate governing rules and molecular components responsible for oncogenic translocations in a cancer model relevant for human disease. In the first aim we will test the hypothesis that -catenin/Tcf-1 and the recombination and DNA repair components participate in common chromatin remodeling complexes that mediate maintenance of genome integrity. In the second aim we will test the hypothesis that in thymocytes with stabilized -catenin TCR signaling enables topological proximity of DNA damaged loci, thus enabling recurrent illegitimate recombinations. The proposed research will define molecular mechanisms responsible for illegitimate DNA recombination and genomic instability in cancers that are causatively associated with deregulation of -catenin.